1. Field of the Invention
The present invention relates to dicing blade hubs for dicing saws and, more particularly, to precision ground dicing blade hubs and method for making same.
2. Background of Related Art
High speed circular hub mounted dicing blades have been used for decades to cut wafers into pieces or die for subsequent fabrication into various semiconductor devices. A typical dicing saw blade is illustrated and described in U.S. Pat. No. 3,691,707.
The dicing saw blade is typically made by turning the aluminum hub of the saw on a high precision lathe. The aluminum hub is gripped in a chuck of some sort and a tool, usually diamond or carbide tipped, is used to remove the excess material and to produce the finished part. This process is both relatively cheap and fast and has been virtually unchanged for 20 years. After the hub has been turned, it is plated on one radial surface with a nickel and diamond grit composition to form a thin membrane of nickel bonded diamond grit which will actually become the dicing blade. This membrane is typically 0.0006-0.004 inches in thickness. To convert the membrane into the dicing blade, part of the aluminum hub is removed by a lathe and a thin aluminum film is left adjacent the membrane. This thin aluminum film is etched away to leave the nickel and diamond grit membrane exposed as the dicing blade. When the membrane is free standing, it is very fragile. The dicing saw blade is now ready to use to dice silicon wafers.
In practice, the dicing blade cuts through the wafer at a rotational speed of about 30,000 rpm. Because the hub surface supporting the dicing blade is not very flat due to the method of manufacture, the dicing blade flexes or wobbles in its rotational orbit. Actual measurements of turned hub surfaces show that the flatness deviation is in the range of 100 to 500 micro inches. Proof of this wobble is evident in the kerf (cutting path) generated by the dicing blade; for example, a 0.002 inch wide blade may create a 0.003 inch wide kerf because of the wobble and related lateral deflection. At a rotational speed of 30,000 rpm, there is a mechanical stress on the edge of the dicing blade. However minuscule the wobble, the rate of flexing is 500 times per second and each valley becomes a potential stress riser. Over a period of time, the flexing causes fatigue along the weakest stress riser and the dicing blade will break. This is one of the major causes for dicing blade failure, particularly during early stages of use. When such failure occurs, it is not unusual to ruin the wafer being cut. The costs attendant with such ruined wafers relate to die loss, the cost of production to date along with lost time associated with replacement of the dicing saw blade and the new wafer.
This method of production inherently has two problems which limit the life of the dicing saw blade. One problem arises because the lathe or turning process typically leaves behind turning marks that manifest themselves on the side of the aluminum hub upon which the dicing blade will be formed. Thus, one side of the dicing blade is rough due to the marks left by the lathe. It is well known in the mechanical art that roughness, grooves, notches or scratches in a piece of material act as stress risers and cause that material to suffer fatigue cracking when stressed and flexed many times. This leads to the second problem which is one of flatness and parallelism of the hub. Although a lathe may be able to turn a part flat while it is held in the chuck, as soon as the part is released from the chuck, the part distorts because of the pressures previously exerted upon it by the chuck jaws. Therefore, it is almost impossible to produce the hub on a lathe to the flatness and parallelism required to prevent wobble during the wafer sawing or cutting process. Rapid flexing of the blade will occur due to the wobble as the wafer is cut. The turning marks on the side of the dicing blade act as notches to accelerate fatigue cracking and bring about a premature failure of the dicing blade.
The traditional lapping method uses flat metal plates set on a rotating table onto which the parts are placed in holders or carriers. As the lapping table rotates, the parts are held in place within a fixed rotating well or truing plate. A slurry, a mixture of coolant and aluminum oxide or diamond grit, flows onto the rotating metal table and laps the parts flat. However, during the lapping process, the grit in the slurry becomes embedded in the part. The embedded grit renders the hub difficult, if not impossible to nickel plate and therefore it is difficult to form the dicing blade on the aluminum hub.